In oil and gas operations (e.g., oil and gas processing plants, wells, offshore rigs, and landfills), situations arise that may necessitate burning of combustible waste gases from time to time. Environmental laws and regulations and ecological considerations require that these gases be burned with a smokeless efficiency before being released into the atmosphere. For example, the United States Environmental Protection Agency (EPA) regulations (40 C.F.R. §60.18) require that a gas flare operate at the destruction and removal efficienty (DRE) of 98% or greater. DRE is the percent removal of hydrocarbon from the flare vent gas. The EPA regulations provides empirically derived equations that use variables such as flare type, the net heating value of the gas being combusted, and the exit velocity of the gas to indirectly determine if a flare is operating at a DRE of 98%. In other words, the EPA assumes that a flaring meeting the operating condition requirements of 40 C.F.R. §60.18 operates at a DRE of 98%. But even if a flare is operating at the regulatory criteria, the actual DRE can often be lower than 98%. So a system that operates in compliance with the regulations can still produce a significant amount of smoke, which may raise environmental concerns.
The gases that need to be flared vary in composition and pressure. High-pressure gases generally have sufficient kinetic energy and do not require additional energy to be imparted to the flaring system to burn smokelessly. But low-pressure hydrocarbon gases, having a hydrogen-to-carbon molecular weight ratio of less than 0.3, 0 produce smoke as a result of incomplete combustion and the formation of free carbon. Various systems and methods for coping with smoke generation have been proposed. One method uses steam or water as smoke suppressant. Others use a supply of high-pressure gas to increase hydrogen-to-carbon ratio. Another method uses forced air to provide turbulent mixing of air with the hydrocarbon gas for complete smokeless combustion. Providing additional kinetic energy and oxygen to the low-pressure gas supply can increase the DRE. This method is typically referred to as “air-assist flaring.” This type of air-assisted flaring is advantageous when neither steam nor assist gas is available because a simple air blower can be used to provide forced air. Some air-assisted flare systems suffer from inefficiencies due to over-assisting, i.e., providing more assist air than is necessary or providing assist air but not actually increasing the DRE.
An oil and gas operation may need to vent both high- and low-pressure gases at various times. Because low-pressure gases require additional assist systems (e.g., steam, gas, or air), separate flare systems are often used for the different pressure waste gases. Some have proposed designs for a single flare system that is capable of processing both low and high pressure waste gases, but these systems have certain limitations and inefficiencies. U.S. Pat. No. 3,822,985 issued to John F. Straitz, III (“Straitz patent”) discloses a flare stack gas burner for waste combustible gases at both low and high pressure with separate delivery systems for each. The Straitz patent teaches a central flare stack with a low-pressure gas delivery tube disposed inside. A separate tube feeds air into the central flare stack to mix with the low-pressure gas with a swirling action caused by the burner tip design. High-pressure gas delivery tube is disposed entirely outside of the central stack, and feeds into a manifold at the burner tip. There are several major inefficiencies associated with the system as disclosed in Straitz patent. First, the blown air must travel the entire inner space/volume of the central stack before it mixes with the low-pressure gas. This results in a significant loss of kinetic energy, which in turn results in smoke formation. Second, a blower with a large capacity and high power must be used to deliver sufficient forced air through the central stack. Third, to be operative, one must use the specialized burner assembly with a plurality of low-pressure and high-pressure apertures organized in a ring fashion at the burner tip. Finally, an existing gas flare cannot be retrofitted to provide for a system that is capable of processing both low- and high-pressure gases.
U.S. Pat. No. 4,105,394 issued to Robert D. Reed (“Reed patent”) discloses a smokeless burner apparatus for single structure flare systems capable of burning waste gases from high and low pressure sources. Like the Straitz patent, the Reed patent teaches a specialized burner assembly with a hub-and-spokes design. The low-pressure gas delivery pipe is again disposed within the central stack. Forced air is blown through the entire inner space of the central stack before mixing with low-pressure gas. High-pressure gas delivery pipe is disposed within the central stack. The apparatus of the Reed patent suffers from several of the same inefficiencies as the apparatus of the Straitz patent.
Another problem faced by previous gas flares is the presence of liquids in the waste gases from the oil or gas production stream or from storage tank battery. Most oil and gas operations have a gas-liquid separator (also known as a “knockout drum” or a “scrubber”) upstream of the flare to remove water or oil from the waste gas being combusted. Often, even with the upstream scrubbing step, not all of the liquid is removed. This can be due to the inefficiencies, overload, or failure of the upstream gas-liquid separator. The free liquids entrained in the gas streams travel and build up in the gas delivery system. This can cause inefficient burning of the gases, and may necessitate cleanup or replacement of gas flare system.
Another problem posed by previous gas flares is the level of noise produced by the system. While some flaring units are used in rural settings where the noise is of little concern, many flaring units are used quite close to residential areas. Furthermore, even in sparsely populated locations, the operators of the flaring units will have to approach the system from time to time. Extended exposure to a high level of noise can be detrimental to human auditory system.